1. Field of the Invention
The present invention is broadly concerned with improved extrusion apparatus generally including a preconditioner and an extruder, with an intermediate feeder screw assembly coupled between the preconditioner outlet and the extruder inlet in order to positively convey material being process from the preconditioner directly into the extruder. More particularly, the invention is concerned with such apparatus and corresponding methods, wherein the feeder screw assembly preferably includes a substantially closed housing with a pair of juxtaposed, intercalated auger conveying screws within the housing having flighting preferably extending from a point closely adjacent the preconditioner outlet and into the extruder inlet.
2. Description of the Prior Art
Extrusion systems are commonly used in the processing of human food or animal feed materials. Generally speaking, such systems include a preconditioner operably coupled with a downstream extruder. The preconditioner is typically in the form of an elongated vessel having internal mixing shafts operable to receive incoming material and to heat, moisturize, and agitate the material for at least partial precooking thereof. The preconditioned material then is conveyed through a simple downspout or the like under the influence of gravity into the extruder inlet. In the extruder, the material is finally processed, owing to the presence of one or more elongated, helically flighted, axially rotatable screws which convey the material along the extruder barrel and through an endmost restricted orifice die. The extruder generates additional heat, pressure and shear conditions, which effect the final cooking and forming of the extrudate.
The use of downspouts or transitions between the preconditioner outlet and extruder inlet can create significant operational and/or contamination problems. For example, owing to the fact that the preconditioner and downspout are substantially at atmospheric pressure, there is a possibility that airborne contaminants are incorporated into the material during processing. Furthermore, because the flow of material in the downspouts relies on gravity and is not positively conveyed, the material can clog within the downspout, thereby disrupting the desirable continuous flow of material between the preconditioner and the extruder.
U.S. Pat. No. 6,482,453 describes extrusion systems designed for quick changeover between different types of materials to be processed. In one embodiment, the '453 patent provides a powered, variable speed, variable output screw-type discharge feeder immediately below the preconditioner outlet. This discharge feeder is designed so as to permit variation in the residence time of the material within the preconditioner, and to assist in the desired quick changeover. However, the system of the '453 patent still relies on gravitational flow from the discharge feeder to the inlet of the downstream extruder.
There is accordingly a need in the art for improved material transfer apparatus between a preconditioner outlet and an extruder, which positively conveys preconditioned material to a downstream extruder, without full reliance upon gravitational flow. Furthermore, it would be desirable if such material transfer apparatus would allow superatmospheric pressure conditions within the preconditioner and transfer apparatus, while also permitting maintenance of temperatures above 212° F.